This invention relates to a turbo engine having a gas turbine to drive propeller or propfan rotors through a gear speed decreaser or speed reduction transmission, where the speed reduction transmission exhibits a separate lubricating oil system comprising an oil cooler, oil pump and oil tank.
A turbo engine of this generic description has been disclosed in, e.g., German Published Examined Patent Application (DE-OS) 37 14 990, where a suitable gearbox oil cooling system is arranged as a compact unit to dissipate the considerable heat loss of the speed reduction transmission. In this arrangement the operability of the cooling system largely depends on a reliable supply of cooling air to the oil cooler to suit all operating conditions of the engine. This design exhibits one special disadvantage; at idle speed or ground operations the energy conversion and the attendant heat generation already reaches a considerable level, while energization of the oil cooler with cooling air is still inadequate.
A broad object of the invention is therefore to provide an arrangement of the above noted type wherein the cooling air supply system provides adequate and reliable oil cooler ventilation at all operating points of the engine this object is achieved according to preferred embodiments of the invention by providing an oil cooler impeller connected to the gas turbine output shaft and disposed to act on the cooling air for the oil cooler. This arrangement provides an advantage in that the supply of cooling air flow to the oil cooler is assisted by means of an additional cooling air impeller, where the delivery capacity essentially varies with the speed of the drive shaft and the outer diameter of the impeller wheel. This arrangement provides--at no more than moderate pressurization--a sufficient supply of cooling air to the cooling air duct inlet also under extreme operating conditions, which typically occur during reverse thrust application upon touchdown of the aircraft, especially when thrust reversal is effected by fan blade variation.
This arrangement also enables the oil cooler to be advantageously designed for higher cooling air pressure losses, and hence to be made smaller and lighter in weight. It simultaneously permits the cooling air requirement to be reduced and adverse effects on the engine cycle to be reduced accordingly.
The number and size of the struts to duct the cooling air in the compressor inlet section can be reduced, because the allowable pressure losses are greater, which makes for a cleaner gas turbine inlet. Another benefit is that the flow areas in the cooling air duct can be reduced, again on account of the higher allowable Pressure losses, leaving more available mounting space in the hub housing.
Since the oil cooler impeller arrangement in especially preferred embodiments of the invention connects directly to the shaft, omitting intermediate gearing, a compact and relatively simple construction for integrating the impeller stage is provided. The absence of intermediate gearing additionally makes for extreme ease of maintenance of the oil cooler impeller.
Owing to the relatively low thermal and mechanical loads the oil cooler impeller can be manufactured as a light-weight construction using light weight, low alloy materials (e.g. aluminum alloys). The weight penalty associated with a centrifugal impeller is thus minimized, as are the resultant inertia forces impairing the dynamic response of the propfan stages.
In a further advantageous aspect of certain preferred embodiments of the present invention, the cooling air duct exhibits a U-shape when viewed in an axial plane, where it extends radially inward from intake holes on the outer circumference of the hub, where the impeller is arranged in the curved region, and where the duct thereafter extends radially outward. This makes for a compact and low-drag arrangement.
In a further advantageous aspect of certain preferred embodiments of the present invention, the impeller is the centrifugal type to give a high pressure difference at maximum compactness.
In a further preferred aspect of preferred embodiments of the present invention, the cooling air duct issues into an outer casing chamber through hollow structs of the gas turbine inlet duct. This prevents the hot air from influencing the gas turbine parameters (raising the high-pressure compressor inlet temperature). In an alternative embodiment the cooling air duct communicates with the gas turbine inlet duct through a number of hollow struts open at the trailing edge. Although this involves some influence on the gas turbine parameters, the pressure rise generated by the fan stage can here be used for propulsion, and the tapped cooling air can gainfully be reinjected into the engine cycle.
In certain preferred embodiments, the intake holes preferably taken the shape of grid screens that have the form of cylindrical sections and are spaced over the circumference of the hub. This makes for a low-drag outer contour of the hub in the region upstream of the gas turbine inlet duct. Within the grid screens, a second, concentrically disposed row of grid screens is preferably arranged, where the inner row of screens can be offset to vary the flow area of the duct. This provides a simple means for controlling the cooling air flow. Simultaneously this arrangement prevents unfavorable reprecussions on the core engine inlet flow at off-design, when the cooling air take-off is reduced.
In an alternative embodiment of the present invention the intake holes are arranged in the interior of the gas turbine inlet duct, the impeller is arranged upstream of the oil cooler, and the coolant flow duct connects to deicing lines downstream of the oil cooler. In this manner the cooling air heated in the oil cooler can be used for deicing the hub, the spinner, and/or the propfan blade sections near the hub. The cooling air duct preferably connects to deicing ducts which extend to the nose area underneath an outer skin and have regularly spaced outlet holes. The escaping cooling air can thus prevent icing of the hub section. The cooling air duct alternatively or additionally connects to deicing ducts on the propfan blade leading edges for further deicing duty.
In a further advantageous aspect of the present invention the oil cooler impeller is disconnectably connected to the shaft, so that the ventilation effort of the impeller can be omitted in the presence of sufficient ram pressure, as in the inflight condition.
Alternatively a shut-off flap can be inserted into the cooling air duct for control of the cooling air flow, or of the impeller ventilation effort.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.